Algae have been cultivated artificially for such diverse purposes as the production of food for animals and humans, the treatment of sewage and waste waters, and the accumulation of radioactive wastes. More recently, algal cultures have been used for the production of enzymes having industrial and research applications and for producing oils and other materials having nutritional value. Modern biotechnology offers an opportunity for the genetic modification of algae to yield cultures capable of producing a wide variety of useful materials.
Various methods and equipment have been employed for the artificial culturing of algae. Perhaps the simplest procedures have involved the use of shallow open ponds exposed to sunlight. Such ponds are subject to contamination by dust, other microorganisms, insects and environmental pollutants and provide minimal ability to control the degree of exposure to light, temperature, respiration and other important factors. A more sophisticated approach has involved growing algal cultures in plastic-covered trenches and ponds, optionally having electrically powered pumps and agitators. These configurations reduce the chances of contamination of the culture and permit more accurate control of temperature, respiration and other parameters. Such configurations are still quite inefficient in terms of providing adequate and uniform amounts of light to the algal cells, particularly when sunlight is the sole source of light.
Unlike other microorganisms, the nutrient requirements of algae are very inexpensive carbon dioxide being the principal source of carbon. On the other hand, the photosynthetic processes require that the algae be exposed to a relatively constant and uniform source of light. A primary design factor for modern photobioreactors involves providing a means for uniformly exposing the cells in the algal culture to the optimum amount of visible light. Like many plants, algae are quite sensitive to the amount and kind of light. Excessive light intensity can damage and kill algal cells. Too little light results in low levels of photosynthesis.
A number of design factors are affected by the means selected for supplying light to the cells. For example, light sources, including natural sunlight, often emit substantial amounts of heat. Algal cultures are sensitive to heat, and many of them grow most efficiently at relatively low temperatures (e.g., about 27.degree. C). Thus, means must often be provided for cooling the algal culture and dissipating heat generated by the light source.
Two design factors closely related to the requirement for a uniform and constant supply of light are the cell density and the light path length. Like conventional fermentation processes, it is usually desirable to use as high a cell density as possible. Many of the same considerations apply to algal cultures as to bacterial cultures. For example, in addition to the light requirements, one must take into account the competition for nutrients, respiratory demands, viscosity and pumpability of the culture medium, and the like. An extremely high cell density results in cells more than a few millimeters from the light source being effectively shielded from the light. Simply increasing light intensity will not overcome this problem, because highly intense light will damage or kill cells near the light source.
The only effective way of increasing cell densities while maintaining a uniform amount of light is to employ a relatively short light path length. Of course, the requirement that the photobioreactor have a relatively short light path length introduces a new set of design problems. For industrial applications, it is usually desirable to employ high-volume microbial cultures. Large culture volumes are amenable to continuous or large-scale batch recovery operations and generally result in economies of scale. Satisfying the requirements for large culture volumes and short light path lengths mandates that the photobioreactor have large, transparent walls which are closely spaced to define a light path and a fluid chamber within which the algal culture is contained. The transparent walls are illuminated with an appropriate light source to sustain the growth and photosynthetic reactions of the cells.
Various designs of such photobioreactors have been employed A relatively simple design which has been successfully used in laboratory and pilot plant operations is simply a glass chamber having large, flat, parallel side walls and a narrow bottom and end walls. A gas sparging tube is placed in the bottom of the chamber to allow carbon dioxide or carbon dioxide-enriched air to be sparged through a culture medium contained in the chamber, and banks of fluorescent light tubes are arranged adjacent to the side walls of the chamber. Inocula, nutrients, buffers, and the like can be introduced into the chamber through the top which may optionally be covered with a lid. This design has been very successful and useful for small scale operations, but scale-up to industrial operations poses numerous difficulties. A primary difficulty is safety-related. The fluorescent lighting must be arranged uniformly along the side walls of the chambers. As greater numbers of chambers are used, the presence of a large number of fluorescent tubes and the accompanying electrical circuitry poses a substantial risks of electrical shorting and electrocution of operators resulting from rupture or breakage of a chamber.
In addition, large banks of fluorescent tubes and their accompanying sockets, mounting brackets, electrical circuitry and the like are quite expensive from both the standpoint of initial capital investment and continuous power requirements. In particular, such designs involve large installation and maintenance costs.
An alternative embodiment of a bioreactor employing a fluorescent tube involves a cylindrical culture chamber having glass walls which surround a single fluorescent tube. The culture chamber may also be surrounded by a concentric cylindrical water jacket for controlling the temperature of the culture. Such a photobioreactor is described by Radmer, R., Behrens, P., and Arnett, L., "An Analysis of the Productivity of a Continuous Algal Culture System," Biotechnology and Bioengineering, 29 pp. 488-4392 (1987). This design has also proven very valuable for laboratory-scale algal culturing operate ions, but, for many of the reasons described above, has not proven particularly useful for large-scale operations.
Thus, in recent attempts to design large-scale photobioreactors, attention has been focused on devising efficient means for distributing light uniformly, and in the correct intensity, across large transparent walls of the reactor. One approach to this problem has been to use fiber optic cables to distribute light from one or more high-intensity light sources to an algal culture medium. This approach is described, for example, by Mori, K., Ohya, H., and Furune, H., "Sunlight Supply System and Gas Exchange in Microalgal Bioreactor System." Advances in Space Research 1986. Eds. R.D. McElroy and A.I. Shoog, Pergammon Press (1987) (in press). A principal disadvantage of using fiber optic cables for large-scale photobioreactors is their cost. To provide a uniform distribution of light over large surface areas, a very large number of optical fibers must be employed. Both the capital investment and the fabrication costs associated with such a system can be prohibitive.
Various photobioreactor designs are reviewed in an article by Yuan-Kun Lee, "Enclosed Bioreactors for the Mass Cultivation of Photosynthetic Microorganisms: The Future Trend," TIBTECH, July 1986, p. 186-189. A significant need still exists for large-scale photobioreactors capable of using high intensity, low-cost lamps which are physically remote from and preferably above the liquid culture medium to minimize electrical hazards and transfer of heat from the lamps to the culture medium.